The present invention relates to a cooling system for a gas turbine for cooling a component of the turbine downstream of a seal and which seal restricts the cooling flow to the component sufficiently to adversely affect the component and particularly relates to apparatus and methods for augmenting the cooling flow to the downstream component.
In gas turbines, a portion of the total air flow from the compressor inlet is diverted to various turbine components for purposes of cooling or providing purge flow to those components. The diverted air can consume a large proportion of the total air flow, for example, as much as 20%. The management and control of these parasitic flows, for example, through the use of advanced seals, can dramatically increase the performance of the turbine. Typically, air under pressure is extracted from the compressor and bypasses the combustion system of the turbine for use as a cooling or purge flow for various turbine components. A cooling flow inevitably flows past seals between relatively movable components. For example, labyrinth seals between rotatable and stationary components are often employed and leakage flows past the labyrinth seals have been used for cooling certain turbine components downstream of the seals. As a specific example, the high packing seal is typically a labyrinth seal and the cooling air leakage flow past that seal is used to purge the downstream wheelspace, as well as to cool the rotor.
With the advent of advanced seals, such as combined labyrinth/brush, a bradable or certain labyrinth seals, used in place of the more conventional seals, the advanced seals may restrict the leakage flow past the seals, to the extent that such leakage flows can no longer provide the necessary cooling or purge flow to the downstream components. That is, advanced seals are designed for the very desirable effect of increasing sealing capacity. However, in some locations the flows in which the seals are to restrict are used for cooling or providing purge flow to turbine components downstream of the seals. If the magnitude of the flow restricted by the advanced seal is too great, the designed temperature limits of the downstream component may be exceeded. Conventional labyrinth seals, for example, do not typically restrict the leakage flow sufficiently to cause high temperatures in the downstream temperature components, i.e., they do not restrict the flow sufficiently to cause the component to approach or exceed its designed temperature limits. Advanced seals which are being increasingly used in turbines in lieu of the more conventional seals, however, may restrict leakage flows sufficiently such that the temperature of the component may approach or exceed its designed temperature limit.
In an embodiment of the present invention, an ejector is employed which uses a primary driving or motive fluid to entrain a lower temperature fluid and thus drop the temperature of the combined fluid. The combined fluid is used to augment the cooling or purge flow to the downstream component. Because of the lower temperature, the amount of fluid required, e.g., to cool the downstream component, is reduced and the advanced seal becomes more effective. Particularly, the motive fluid may comprise an extraction from the compressor which is mixed in the ejector with a suction fluid from an earlier lower pressure and temperature stage of the compressor. By accelerating the motive fluid, dropping its static pressure and combining the motive fluid with the suction fluid and passing the combined flow through a diffuser, the resulting flow is at a temperature intermediate the extraction air temperatures. By using this lower temperature air augmentation, less air is needed to maintain the desired temperature limit and thus advanced seals can be used in locations where individually they would cause excessive temperatures in the component due to the restricted air flow. Also, less of the more valuable later compressor air and more of the less valuable earlier compressor air are used to cool or provide purge flow to the downstream component. The cooling flow is thus minimized and improved performance is achieved without sacrificing part life.
In a preferred embodiment according to the present invention, there is provided a method of cooling a component of a turbine or providing a purge flow to a space downstream of a seal comprising the steps of restricting a supply of cooling or purge air flowing past the seal to the downstream component such that a predetermined temperature limit of the downstream component or space is exceeded, extracting a first flow of air from a stage of a compressor associated with the turbine at a first temperature, extracting a second flow of air from another stage of the compressor at a second temperature lower than the first temperature and combining the first and second flows with one another to provide a third flow of air to the component or space at a temperature intermediate the first and second temperatures to cool the component to or provide purge flow to the space at a temperature below the temperature limit.
In a further preferred embodiment according to the present invention, there is provided a cooling system for a turbine comprising a turbine seal, a turbine component and a passage in the turbine for carrying cooling medium past the seal along the passage to the component, the seal restricting the flow of the cooling medium along the passage to the component such that a temperature limit of the component is exceeded, a first flow path for flowing cooling medium from a pressure stage of a compressor associated with the turbine at a first temperature, a second flow path for flowing cooling medium from a stage of the compressor at a second temperature lower than the first temperature and an ejector for mixing together the flows of cooling medium from the first and second flow paths to provide a mixed flow having a temperature intermediate the temperatures of the flows along the first and second flow paths and a passageway for receiving the mixed flow and combining the mixed flow and the flow of cooling medium along the passage for cooling the component.